A fundamental finding in the conditioning literature is that reinforcers have a relative, not an absolute, effect of behavior. Multiple-schedule behavioral contrast provides one example of reinforcer relativity. The rate of responding in one component of a multiple schedule varies inversely with the conditions of reinforcement in the other component. Eight sets of experiments will employ operant techniques to test the idea that differences in the value of the reinforcers between the baseline and contrast phases contribute to multiple-schedule contrast. Differences in reinforcer value might occur because the baseline and contrast phases provide different numbers of reinforcers. As a result, different amounts of habituation may occur to those reinforcers. If this theory is correct, it will revise our understanding of contrast and of reinforcer relativity. Eventually, dynamic changes in the effectiveness of reinforcers may contribute to understanding other phenomena such as spontaneous recovery, preference for variability and the regulation of motivated behavior. Because operant principles are often used to understand and to treat health-related problems (e.g., drug taking, obesity), this research may have many implications for both theory and practice in the area of health.